1. Field of the Invention
This invention relates to a disc-shaped recording medium (thereinafter referred to as "disc") capable of recording data signals by means of a light beam, particularly to a disc possessing a concentrical or spiral groove formed in advance to serve as a recording track (hereinafter referred to as "pregroove"), which is characterized by the presence of additional signals such as an address data signal stored in advance on the pregroove.
2. Background of the Invention
A disc has been devised for use in a recording/reproducing apparatus adapted to record and reproduce data signals by means of a light beam. This disc possesses a pregroove formed in advance to serve as a recording track and has an address data signal identifying the pregroove and stored in advance in the shape of surface irregularities on the pregroove.
FIG. 1 is a model diagram illustrating in cross section a conventional pregrooved disc having an address data signal identifying the pregroove and stored in advance on the pregroove. In FIG. 1, 4 denotes a disc substrate made of a material such as transparent resin, 1, 2 respectively denote an address data part and a pregroove part formed on the disc substrate, and 3 denotes a recording medium layer. FIG. 2A is a plan view illustrating in the form of a model the recording medium layer 3 of the aforementioned disc.
In the conventional pregrooved disc, the address data has been formed in conjunction with the pregroove by intermitting the pregroove as illustrated. Thus, the depth of the pregroove part 2 and that of the address data part 1 have been identical.
When this pregrooved disc is rotated and the laser beam is caused to track the pregroove and keep impinging thereon and the reflected laser is read out, there is reproduced an address data signal in accordance with the surface irregularities of the address signal part 2 as illustrated in FIG. 3.
FIG. 2B is a cross-sectional diagram taken through FIG. 2A along the line 2B--2B, illustrating the pregroove part 2 and the address data part 1 as recorded in the form of surface irregularities on the disc substrate 4. They correspond to the reproduced address data signal and the pregroove illustrated in FIG. 3. As readily noted from FIG. 3, the address data signal recorded in conjunction with the pregroove on the conventional disc forms rectangular waves as illustrated in FIG. 3. The spectrum of the signal, therefore, is distributed in a very wide range. The spectrum of the reproduced address data signal illustrated in FIG. 4A, for example, is known to be distributed up to a very high frequency as illustrated in FIG. 4B.
When a signal is recorded on the conventional pregrooved disc possessing an address data signal, therefore, it is difficult for the data signal to be recorded by being superposed on the address data part 1. Even when the data signal is recorded somehow or other, it will be found difficult during the subsequent reproduction of the data signal to separate the address data signal recorded in advance and the data signal so recorded thereover.
Further, since the address data signal has been formed in conjunction with the pregroove by intermitting the pregroove, the tracking signal which is detected using the pregroove is degraded. Consequently, the signal to noise ratio of the tracking signal becomes lower.